I always figured the main way to tell a male from a female was its agressiveness, and its finnage.

I had a sorority tank of many bettas, all allegedly female (by the LFS), and as their numbers thinned for a various number of reasons, I was left with my 5 favorite.

I recently purchased a 28 gallon tank and started to move the members of the sorority and the plants in it over to this 28 gallon.

As time wore on, the agressiveness of one of the females really started to become more apparant. Its fins also grew substantially longer than the rest of the females. This female's fins look to be nearly as long as the majority of males.

I am aware that there are a number of female betta variants that have long fins, but this one's fins just grew and grew.

Are some bettas hermaphroditic like clown fish? I really woudl like to know, because it seems now that this last betta is by itself in its own tank, it has developed a monstrously different personality.

In being by itself it finds activities of its own such as more leaping in the tank, picking up pebbles and dropping them below. Inquisitive stares at me, and a desire to rearrange the decorations on its own.

Could it be, that all along, this betta is in fact a male, and was not properly removed by the breeder from the females?

Could you take a picture of her/him? Some females can be very aggressive; sometimes more aggressive than males. You could've purchased her/him when it was very young and now it's growing fins as it grows older. I would remove him/her right away until you know for sure.

A quick, easy, but not always accurate, way to tell males and females apart is a spot called an "egg spot". All females have it, but it's not always accurate because sometimes males have it. However, for a simple way to MAYBE end this mystery, you might want to look for it. It's on the belly on the betta, and it's fairly easy to notice.
Here's a picture of one...
Do you see what I mean?
If your betta does have it, it doesn't mean it's positively a female, but if it doesn't have it, it's a male. Understand? I explained it confusingly, sorry.

It doesn't have an ovipositor. But its fins are REALLY long, so it might just be hinding it. I tried picking her up (she/hes somewhat docile with me, just agressive against the other bettas in the tank)